This disclosure relates generally to coal power plants, and more particularly to coal power plants having increased combined cycle efficiency.
The efficiency of burning coal to generate electricity can be increased through the utilization of coal turbines. Various minerals in the coal such as, but not limited to, inorganic minerals, however, should be removed to prevent damage to the turbine blades and other system components. The minerals, therefore, are removed either before combustion or after combustion, but prior to the combustion gas entering the turbine. In the former approach, the inorganic mineral content of the coal is reduced to less than about 0.1 weight percent (wt %). Coal of this purity is referred to as ultra clean coal (UCC). UCC can also be utilized as a raw material for the production of a vast array of useful organic compounds and polymers.
The inorganic minerals present in the coal will depend on the coal's source, and can include kaolinite, quartz, clay, such as chlorite and montmorillonite, pyrite, anatase titanium dioxide, and siderate. The inorganic minerals can also include hematite, natrojarosite, dolomite, apatite, flourapatite, feldspar, and gypsum.
One method of producing UCC is by leaching out the minerals by washing the coal with leaching agents, such as strong acids. Methods for leaching to produce clean coal are well known. Examples of leaching agents include hydrofluoric acid, nitric acid, and ferric nitrate. In a specific method, the coal is leached first with an aqueous solution of hydrofluoric acid, then with an aqueous solution of nitric acid or ferric nitrate. Leaching of coal with an aqueous solution of hydrofluoric acid is referred to herein as fluoride treatment.
The spent leaching solution not only contains hydrofluoric acid, nitric acid, or ferric nitrate, but also the minerals leached out of the coal. The mineral impurities present in the coal are dissolved by the leaching agents, thereby releasing silicon, aluminum, iron, titanium, potassium, calcium, sodium, magnesium, barium, strontium, vanadium, copper, manganese, zirconium, zinc, cerium, or a combination comprising at least one of the foregoing elements into the leaching agent solution. The elements and amount of elements released into the leaching agent solution depend upon the source of the coal and the mineral content therein.
The spent leaching solution is a wastewater stream, which should not be released directly into the environment without treatment. One way of treating this wastewater is to boil off the water, and landfill the solid residue. However, coal turbine plants suffer an energy efficiency penalty for boiling off the water from the wastewater. Including the energy to boil off the water, the combined cycle efficiency of a coal turbine plant, which can be measured in terms of higher heating value (HHV), is only about 33%. Such a low efficiency makes burning UCC for power both environmentally and economically undesirable.
An improved method for treating the wastewater streams of coal cleaning processes is desirable. Specifically, it would be advantageous to have a method to remove water from the wastewater without suffering the energy efficiency penalty associated with boiling the water.